Ductile cast iron, also known as nodular iron or spherulitic iron, is cast iron in which the graphite is present as tiny balls or spherulites, instead of as flakes normally present in grey iron, or instead of compacted aggregates present in malleable iron.
The composition of unalloyed ductile iron is similar to that of grey iron, containing similar amounts of carbon, silicon, manganese, and phosphorus. The spheroidal graphite structure is produced by the addition of one or more elements to the molten metal, such elements commonly being referred to as nodularizing agents; on a commercial basis the agent is magnesium and/or cerium.
Ductile iron can be produced as-cast, or given an annealing treatment such as a ferritizing anneal, or can be quenched and tempered. The microstructure of as-cast ductile iron is pearlitic in the matrix along with a small amount of cementite, and has considerable ferrite surrounding each graphite nodule (commonly referred to as a bulls-eye ferrite configuration). The relative amounts of pearlite, ferrite, and cementite are dependent on the composition, type of inoculant, inoculation practice, and, most importantly, the cooling rate.
The microstructure of annealed ductile cast iron, particularly in the case of ferritized annealed cast iron, is a ferrite matrix in which are nestled graphite nodules along with a small or negligible amount of cementite. The microstructure of austempered ductile cast iron is a mixed phase matrix composed of austenite and martensite or bainite (see U.S. Pat. Nos. 2,324,322 and 3,860,457). The microstructure of quenched and tempered ductile iron is tempered martensite and/or bainite (see U.S. Pat. No. 3,702,269).
Each of these types of ductile cast iron microstructures leaves something to be desired in terms of the total combination of physical characteristics. For example, in a conventional as-cast ductile iron the yield strength is typically about 60 ksi, the tensile strength is about 80 ksi, accompanied by an elongation of about 3%. This type of iron is not particularly strong nor is it particularly ductile. An annealed ductile cast iron, particularly one having been subjected to a ferritizing anneal, will have a yield strength of about 40 ksi, a tensile strength of 60 ksi, and an elongation of 10-18%. This latter iron is not particularly strong, although excellent is ductility. A conventional quenched and tempered ductile cast iron will typically have a yield strength of about 90 ksi, a tensile strength of 120 ksi, and an elongation of 2% or less. The quenched and tempered ductile iron is exceptionally strong but poor in ductility.
What is needed by the prior art is a method and ability to produce ductile iron with an improved combination of physical characteristics, including a yield strength of at least 80 ksi, a tensile strength of at least 140 ksi, an elongation of 6-10% as well as exceptional hardness in the range of 275-290 BHN.